When I first joined about 3 months ago I posted about how my 10g tank wouldn't finish cycling normally, after I mistakenly washed the filter sponge too soon after the cycle was completed. The nitrites kept showing up although ammonia and nitrate levels were fine. I was advised to add one sponge (which I did) and do water changes every 2-3 days until it evened out.

Thing is, it never evened out, no matter what I do! Sometimes it seems to be setting, then it goes wacky again! I have 6 pygmy cories left, lots of live plants, the Betta was moved to his own 5g home a couple weeks ago, and I was hoping it would help the cycle finally set, and I thought it did as for a week thing were pretty stable...then it spiked again after a 30% water change! The fishes seem pretty unmoved (they even laid eggs several times!). I would suspect a faulty water test kit...but I use Hagen's liquid test and the kit was bought recently so I doubt it. I'm patient and I don't mind constant water changes and testing, but I'm starting to feel like I'm doing all of this for nothing! I also vaccuum about 20% of the bottom every week (not more to keep the bacterias). I was told a lot of live plants could be harmful to water chemistry, could that be true? I don't understand my tank at all

Something doesn't sound right to me. Are you certain that you are not confusing nitrite with nitrATE readings? nitrite poisoning would probably have killed the corys by now. Are the results you get before or after water changes? If after how long after? What water conditioner are you using? How often do you feed the corys?

Nope definitely nitrite, and yes I am as surprised as you, corys in nitritey waters aren't supposed to spawn like crazy, and yet they are!

I feed them morning and evening. I test every day regardless of water changes, but my latest result is quite high, TWO DAYS after a 30% water change and vaccuuming. I use AquaPlus conditionner.

I think it would help to feed only evenings as they are not as active in the morning and I've noticed that morning food is often left until they 'wake up', but other than that, I don't get what I'm doing wrong!

cories respond to fresh cooler water and/or high protein diets. this could be the spawning factor. its is weird that nitrites show but the other two don't. although nitrite is less toxic than ammonia, its still a potential killer. as far as the plants. that is completely false. having too many plants will not screw up your water. plants take from the water column. now if they are rotting and decaying then yes it would add waste.

Have you tested your tap water, not that I have heard of anyone having nitrites in there tap water. Only feeding them once a day could not hurt. During morning feedings, i wait about 1/2 hr after turning the lights on to feed them, gives them a chance to wake up. Not sure what the problem with your cycle is, but I do hope it gets better for you soon!

Not sure if you have any fry in the tank, they need to eat a few times a day.

I checked my plants and none are rotting, but I removed a few just to make sure. I regularly have dead leaves floating but I remove them as soon as I see them.

Something I am wondering is, in accordance with what I was advised, I didn't vaccuum the bottom of the tank at all during the month of cycling, and then not long after I added fishes, I threw my cycle out of whack so I stopped vaccuuming again for a while. Could it be that waste from plants could have accumulated on the bottom (I have half-gravel, half-sand bottom) during cycling and is causing nitrite spikes? I only recently started vaccuuming regularly since I was afraid to damage my cycle even further if I vaccuumed the bacterias in the substrate. before that I mostly vaccuumed every 2 weeks or so.

I wouldn't worry about the fry, they probably got sucked in by the filter :/ I didn't really want fry since they'd add even more waste into my cycle!

Sorry ,had a bit of emergency here at work and had to cut short previous post. As stated i would switch to PRIME water conditioner. It will detoxify Ammonia, chlorine, and chloramines as well as nitrites.You state that the fish do not appear stressed which leads me to suspect that something is affecting the test results considering you record no ammonia but high nitrites and some nitrates. I might also find a fish store that would test a sample of your water preferably a store that uses similar test kit such as API freshwater master kit which is used by many and far more accurate than test strips.There are many reasons that test results can sometimes be less than accurate. Not waiting for the prescribed length of time to compare color charts, waiting too long to compare color charts,not holding the test solution vertical so that drops are uniform, not using enough water and or solution, using expired test kits,etc.
Also, reducing feeding to once every other day or every two days would help and cutting back on amount offered as well.
Layoff cleaning the gravel, don't clean or replace the filter. If filter or sponge gets clogged, swish it around in aquarium water NOT tapwater and stick it back in.
Layoff vaccuming the gravel. IF you aren't overfeeding there is no need to.
Keep the sand stirred or sifted regularly so toxic gases do not accumulate in the sand which can be harmful to fish.
Don't add anything to the water except dechlorinator. No stress this, cycle that, bio,,,ammo,,, etc. many of these products can affect test results and most ,,if not all, are not needed.
I do not know what your exact test results are for they were not posted but,,, If you will adopt some or all of the above things should become easier for you.

The only likely way plants can harm your aquarium is if you add the wrong ones, either bog or flat out terrestrials that die and decay in the water. True aquatic plants are among the absolute best things you can add to your aquarium and greatly benefit your water quality.

I'd really consider getting a second opinion on the test results. Small fish like pygmy corys are the first to suffer when exposed to things like ammonia and nitrites. I certainly wouldn't expect spawning even with water changes and a good diet just due to the stress of the nitrites.

A good LFS should be able to test your water for you, insist that they don't use strips. With such an odd set of events I'd confirm the test results before proceeding.

Switching to Prime would be a good idea. If you do have nitrites that will help reduce their impact on the fish while you sort out what's going on.